The present invention relates to an information processing system in which a plurality of storage systems are connected by a network, and it also relates to a technique for data transfer and disaster recovery in an information processing system.
Temporary suspension of business, data loss, and the like can occur in an information processing system that contains a storage system when a failure occurs in the information processing system due to a power failure, a fire, an earthquake, or the like. In order to prevent such situations, the same type of information processing system is placed in a remote location which is not affected by fire, earthquake, or the like, and duplicate information is prepared by transferring the data written in one information processing system (hereinafter referred to as the main site) to the information processing system allocated at the remote location (hereinafter referred to as the sub-site). A technique exists for performing this transfer and duplication of data using a network (hereinafter referred to as remote copying) in order to obtain these effects.
The term “remote copying” refers to the transfer of data stored at a main site from the main site to a sub-site. Backing up of data at a remote location, business continuity, and disaster recovery can thereby be performed.
Remote copying includes two types of methods: synchronous remote copying and asynchronous remote copying. In synchronous remote copying, a storage system at the main site returns, to a computer (hereinafter referred to as a host), a response to a write request from the host after data transfer to the sub-site is completed. There is, therefore, no data loss in synchronous remote copying, and the consistency of the data is ensured. However, as the line delay between sites increases, an I/O delay occurs in the main site between the host and the storage system.
In asynchronous remote copying, the storage system at the main site performs data transfer to the sub-site after returning, to the host, a response to a write request from the host. A decrease in I/O performance between the host and the storage system is thereby less likely to occur even if there is a long distance between sites, but the possibility of data loss occurring increases in comparison to synchronous remote copying, and the sequence of the data is not ensured.
Assurance of data consistency in asynchronous remote copying is described in Japanese Laid-open Patent Application No. 2002-149499. Specifically, a method is disclosed in this publication whereby additional information is attached to the written data from the host, and a sorting of the data is performed at the remote system based on the additional information to ensure consistency.
A technique called NanoCopy is also described in “The Hitachi NanoCopy Advantage”, [online], June 1999, Hitachi Data Systems Corporation, Internet <URL: http://www.hds.com/pdf/wp134_nanocopy.pdf> as a method for ensuring the consistency of written data in asynchronous remote copying across a plurality of storage systems. NanoCopy suspends the asynchronous remote copying of a plurality of storage systems at a certain time and creates a replica of the volume at a certain time. By regularly repeating this operation, there continually exists a replica as a volume having consistency at some future time.
A method is also disclosed in Japanese Laid-open Patent Application No. H7-72981 for acquiring a replication of the volume at a certain time at high speed within the same storage. Volume replication by the method disclosed in this publication will be referred to hereinafter as a snapshot.
In the method disclosed in Japanese Laid-open Patent Application No. H7-72981, a volume used for saving data (hereinafter referred to as a volume pool) is secured in advance. The writing performed in the replication source volume subsequent to the replication command is then processed according to the steps described below.
(A) It is confirmed after the replication command whether the writing constitutes the first update for the relevant data area. Step B is executed if the writing is the first, and step C is executed if the writing is not the first.
(B) The contents prior to updating of the data area to be written to are copied to the volume pool, the correspondence information of the replication source area to the area of the volume pool targeted for copying is stored, and step C is executed.
(C) The replication source volume is updated.
When data is read from the replication target after the replication command, processing is performed according to the following steps.
(D) It is confirmed using the correspondence information whether the area, for which there was a read request, has been copied to the volume pool; and, when it has been copied, step E is executed, and step F is executed when it has not been copied.
(E) The data prior to updating is returned from the volume pool using the correspondence information.
(F) The data is returned from the replication source volume. Replication by a snapshot can create replication with a smaller volume capacity than is achieved in volume replication by mirroring.